The New Orleans Times-Picayune reports today that the New Orleans District of the Corps of Engineers will opening the Bonnet Carre Spillway on Monday to divert floodwater that may threaten the levees at New Orleans. The spillway, bracketed by guide levees, diverts floodwater to Lake Pontchartrain. It was last used during the 2008 flood. Yesterday the paper reported on the Morganza Floodway, which diverts water to the Atchafalaya Basin and which the Corps may open later next week. Meanwhile, Len Bahr at LaCoastPost is discussing the effect these diversions will have on the coastal region.
Bonnet Carre Structure: Floodway
St. Charles Parish, Louisiana
Following the Flood of 1927, the Corps of Engineers abandoned its levees-only policy that had governed flood control since the flood of 1882 and adopted Charles Ellet’s plan to relieve flooding on the Mississippi through floodways, artificial distributaries. The Corps designed a project flood, one that flowed past Cairo, Illinois at the mouth of the Ohio at 2,360,000 cubic feet per second and past the Old River at 3,000,000 cubic feet per second. The Corps constructed four floodways: the Bird’s Point to New Madrid Floodway, the Morganza Floodway and the West Atchafalaya Floodway, which feed into the Atchafalaya River, and the Bonnet Carre Floodway, which feeds into Lake Pontchartrain. Of the 3,000,000 cubic feet per second streaming past Old River, the West Atchafalaya and the Morganza Floodways would siphon off half, leaving the other half to continue on down to the Bonnet Carre Floodway, where an additional 250,000 cubic feet per second would be diverted to Lake Pontchartrain through structure a 7,000 feet long into a six-mile long spillway hemmed in by guide levees.
The Corps built the structure downstream and on the opposite bank from Bonnet Carre Point, located thirty-three miles above New Orleans, where the Mississippi takes a sharp right turn. This is the place where four times between 1849 and 1882, the Mississippi broke through its left bank and created a crevasse. In 1849 the river flowed for six months through a crevasse 7,000 feet wide. The Corps chose a site downstream from the Bonnet Carre crevasse where the soil was firmer and could support the massive structure.
The Bonnet Carre structure is in essence a huge stop-log structure. The Corps set 7,000 timbers side by side in a concrete structure that forms a dam across the main line levee. Once the engineers decide to open the spillway, two cranes run along the top of the structure and pull the timbers out one at a time and lay them across the top. The process takes thirty-six hours. If it needs to be done more quickly, the Corps can release twenty timbers at a time and open the spillway in three hours.
Since its completion the Corps has opened the spillway about once every ten years, sending Mississippi water into Lake Pontchartrain, which then flows into Lake Borgne. Though the initial response to the influx of freshwater is negative, for several years after both lakes experience increased levels of oyster, crab, and fishery production. However, the nine million cubic yards of sediment that get left behind in the spillway has to be removed. When the Mississippi runs high, but the spillway is not needed remains closed, water leaks through between the timber at a rate of less than 10,000 cubic feet per second, stimulating the natural cycle of spring flooding to the Lake Pontchartrain estuary.[i]
[i] U.S. Army Corps of Engineers, New Orleans District, “Bonnet Carre Spillway,” http://www.mvn.usace.army.mil/pao/bcarre/bcarre.htm.